This invention relates generally to the field of electronic device manufacture. In particular, this invention relates to the manufacture of electronic devices using air gaps to reduce capacitive coupling between conductors in such devices.
Advances in integrated circuit technology have reduced the spacing between the metal lines on any given plane of an integrated circuit. Such spacing is now in the sub-micron range. Reducing the spacing between conductive members in the integrated circuit results in an increase in capacitive coupling between nearby conductive traces. This increase in capacitive coupling causes problems, such as greater cross-talk and higher capacitive losses.
Conventional dielectric materials have a dielectric constant in the range of 3.5 to 4.2, e.g. silicon dioxide has a dielectric constant of 4.2. Low dielectric constant (“low-k”) materials are being developed as a replacement for conventional dielectric materials used between conductors on a given layer and between layers. These low-k materials reduce capacitive coupling between the conductors as compared to conventional dielectric materials. An example of such a low-k material is porous silicon dioxide, i.e. a silicon dioxide film having pores or voids within the film. The pores or voids may contain a vacuum or be filled with air or other gas. Typically, the low-k materials have dielectric constants in the 1.8 to 3.0 range. However, low-k materials have not been used in all applications as they can possess severe processing, cost and materials problems.
The lowest possible, or ideal, dielectric constant is 1.0, which is the dielectric constant of a vacuum. Air is almost equivalent with a dielectric constant of 1.001. Accordingly, attempts have been made to fabricate semiconductor devices with air gaps between metal leads to reduce the capacitive coupling between the electrically conducting members. The air gap forming techniques that have been developed have varying degrees of complexity but typically employ a material disposed between metal lines that is subsequently removed to provide an air gap. However, these techniques are not without problems.
U.S. Pat. No. 5,461,003 (Havemann et al.) discloses a method of forming air gaps in a multilevel interconnect structure. This method disposes a removable solid layer, such as a photoresist, between metal lines and then disposes a porous layer over the removable solid layer. The removable solid layer is subsequently decomposed with the decomposition products passing through the porous layer. The removable solid layer is typically decomposed in an oxygen-containing atmosphere, and in particularly an oxygen plasma. This method may be detrimental to other layers in the structure, particularly organic low-k dielectric materials used elsewhere in the structure. Further, photoresists contain a variety of components which make control of the decomposition temperature difficult. Thus, photoresists may take a long time to be removed entirely and/or require harsh conditions to ensure complete removal of all the photoresist components. This patent fails to mention any residue levels following removal of the solid material.
U.S. Pat. No. 6,165,890 (Kohl et al.) discloses a method of forming air gaps in electrical interconnects using polycycloolefin polymers, such as polynorbornene polymers, as the air gap forming material. However, such polycycloolefin polymers are expensive to make and may require relatively high decomposition temperatures, e.g. 380 to 450° C. Such temperatures may be detrimental to other materials used in the fabrication of electrical interconnects. In addition, polycycloolefin polymers are typically prepared using metal catalysts which may contaminate the polymer and result in metal ion contamination in the air gap formed from these polymers.
There is a continuing need for air gap forming materials that can be easily applied to a structure, that can be removed leaving little to no residue, and that can be removed under mild conditions that are compatible with a broad range of materials.